In the printed circuit industry, photographic masks bearing an image corresponding to an electrical circuit, typically represented by a series of lines of dots at a high resolution, are known as phototools. Phototools are typically made using a computer-aided design (CAD) system to prepare data for an exposure apparatus (for example, a photo-plotter) based on a target blueprint or data. Then, this data is used to perform direct writing of a designed pattern (for example, a circuit pattern) onto an emulsion photographic dry plate, which has been prepared by forming a film surface of a photosensitive emulsion layer on an optically clear substrate (e.g., a glass substrate, fused silica or polyethylene terephthalate (PET), polycarbonate, or poly(methyl)methacrylate substrate). The image often consists of many fine lines and junctions spaced closely together, and is typically present as an image layer disposed on a transparent substrate, although it may be disposed within the substrate in some cases. During its use to make printed circuit boards, the phototool is placed face down on a photoresist layer used to manufacture the printed circuit board, and a contact print is made by exposing the photoresist to high intensity light through the phototool. In this way, a single phototool can be used to make multiple contact prints.
Photoresists on which the phototool is placed are usually laminated on sheet copper and small burrs or rough edges of the copper sheet can cause scratches as the phototool is transferred from one photoresist to the next. The phototool is also frequently wiped with a soft cloth to make sure it is dust and lint free. Small particles of dirt can cause scratching as they are wiped across the phototool surface. Because of this general wear and tear on the phototool surfaced during normal use, the phototool must be frequently inspected by microscopy to ensure line continuity. Depending upon the size and the intricacy of the phototool, such microscopic inspections can take 2 to 3 hours.
Since typical phototools are vulnerable to scratching, and since abrasion is a serious problem during normal use of a phototool, protective films and overcoats are often employed to protect the phototool. For example, polyester films coated with various kinds of pressure-sensitive adhesives have been laminated to image layers of phototools to protect the image. Because of their thickness, however, laminating films can cause optical distortion and hence loss of resolution. Thinner protective coatings can be obtained by coating the surfaces of phototools with liquid compositions. After application, the thin liquid coating is hardened to yield the desired protective coat. Epoxy silanes and acrylate esters (for example, polyurethane acrylates) are useful in such coatings because of their resistance to abrasion. Many protective overcoats have limited release properties, however, and may tend to stick to the surface of the photoresist, particularly when relatively sticky materials such as high viscosity solder mask inks are present. As a result they can be damaged during removal from the photoresist.